The present invention relates to an improved form of gypsum wall board, in which the paper cover sheets have improved resistance to delamination and to longitudinal splitting of the board, particularly, when the board is subjected to handling in post-manufacturing operations, particularly in high humidity environments. The preferred embodiments of the present invention provide a gypsum wall board with improved resistance to xe2x80x9cRoll-upxe2x80x9d wherein at least one of the paper cover sheets is made with neutral size and a small amount of a cationic polyamide resin is added to the stock used to make the cover sheet.
As is well known, paper covered gypsum wall boards are made by enclosing a mass of plastic water-gauged gypsum composition between paper sheets and allowing the resulting product to setup; whereafter it is dried and cut into sheets of the desired size. One of the essential features of such a board is a firm adherence of the central gypsum core to the cover sheets, so that the board will not delaminate at the interface between the core and the cover sheets.
This has been quite satisfactorily accomplished in the past by the addition of farinaceous materials to the core. These farinaceous materials contain a certain percentage of a water-soluble starch which, during the drying of the gypsum board, will migrate to the interface between the core and the cover sheets, and will hence be closely associated with the gypsum crystals which interlock with the fibers of the cover sheets, thereby preventing their release. The presence of the farinaceous material also serves to protect the fine gypsum crystals from becoming calcined during the drying of the board, for if they did so, they would lose their strength, with resulting destruction of the bond between the core and the cover sheets. When such loosening of the bond occurs, the resulting boards are known as xe2x80x9cpeelers.xe2x80x9d The gypsum core is thus exposed, and the board, at least to the degree that the peeling extends into the board, is rendered useless, and must be trimmed down to obtain pieces of board of a useful size.
Another type of defect, xe2x80x9crollersxe2x80x9d, can make its appearance, in the general nature of xe2x80x9cpeelersxe2x80x9d, is the result of the delamination of the cover sheets themselves, which split between the plies of paper.
A defect similar to a xe2x80x9crollerxe2x80x9d and in the general nature of a xe2x80x9cpeelerxe2x80x9d is called a xe2x80x9cRoll-Upxe2x80x9d. A xe2x80x9croll-upxe2x80x9d is a post-manufacturing problem, similar to the xe2x80x9cpeelerxe2x80x9d problem, which occurs when paper covered gypsum board is subjected to certain post-manufacture operations. The problem frequently appears in the manufactured housing area, wherein a vinyl layer is adhered to the face side of a gypsum board using a laminating process. Because of the manipulation of the gypsum board in conditions encountered in the lamination process (high humidity and temperature), one or more plies of the 6 or 7 ply paper on the back of the board can become damaged. The xe2x80x9cRoll-Upxe2x80x9d damage occurs when portions of one or more plies of the backing paper separate from the underlying plies of the backing paper and create various defects including the formation of small, tightly packed rolls, not unlike a cigar, on the back surface of the gypsum board. The cigar-like rolls and other defects on the back of the gypsum board can damage the vinyl surface when the laminated boards are stacked after the laminating operation. The xe2x80x9cRoll-Upxe2x80x9d problem involves the delamination of paper plies, not the separation of the paper backing from the core of the gypsum board.
The cover sheets usually employed in the production of gypsum core boards consist, in the case of wallboard that is intended to form the internal walls of rooms, of two types. The cover sheet used on the face side of the board is in the form of a paper built up of a plurality of plies, the two outer plies being usually of a somewhat better grade of recycled paper, such as fly-leaf and newsprint paper. The furnish is a mixture of ground wood, thermal mechanical and sulfite pulps for the outer plies. The inner plies, including the one which will be in contact with the gypsum core of the board is usually made of repulped newsprint and pre-consumer corrugated boxes. The paper used to line the backside of the board is usually made of a plurality of plies of repulped newsprint and post-consumer corrugated boxes. In any event, the papers are made on what is known as a cylinder machine, the pulp being picked up on rotating screens, so that a plurality of layers or plies will be obtained, which, by being superimposed upon each other immediately after their formation will bond to each other and produce a substantially uniform piece of paper. Under certain circumstances, such paper is subject to separation of the plies from each other, i. e. it may delaminate.
When gypsum board is covered with multiple ply paper is dried, there appears to be set up in the paper rather severe strains. Water vapor is absorbed and relaxes the severe strain when the gypsum board is shipped into areas of high humidity and temperature. By relaxing the strain, in severe cases, even the longitudinal splitting of the ply which is nearest to the core may occur. This may occur on either the front or the back of the gypsum board, and is probably attributable to the development of undue strain in the plies, of the paper during the drying of the board.
A factor of great importance is the strength of the bond between the gypsum core and the paper cover sheets, so that the paper cover sheets will not xe2x80x9cpeelxe2x80x9d or de-laminate from the gypsum core. A weakness of the interface bond will produce a totally unacceptable wall finish. Additionally, it is necessary that the paper cover sheets, which are conventionally manufactured, do not delaminate between their adjacent plies.
With reference to the paper cover sheets themselves, they must be of uniform strength throughout with strong bonds between the plies. Additional wet strength in the paper cover sheets make it possible ship gypsum board panels into high humidity and temperature regions without post manufacturing xe2x80x9crollerxe2x80x9d defect problems.
The prior art describes a wallboard in which at least one ply of the paper cover sheet incorporates a cationic polyamide type resin to supplement the conventional rosin and alum size in order to address the xe2x80x9cpeelerxe2x80x9d problem for xe2x80x9chigh suctionxe2x80x9d gypsum board panels. These high suction panels are used as a base for the application of conventional plaster. Gypsum board panels used as the substrate for the application of plaster are required to absorb high amounts of water from the plaster in order to allow the plaster to set.
However, the addition of polyamide type resin, which is compatible with the rosin and alum size, does not change the high suction properties of the gypsum board. The alkaline salt that is added to the paper plies does prevent the migration of the size into the cover sheet to maintain the high suction properties of the paper. At times the presence of the size at the surface of the gypsum board produces a slick bond, which means the crystals of gypsum plaster at the interface do not penetrate the xe2x80x9chigh suctionxe2x80x9d paper and the bond between the paper and the plaster is greatly impaired. This problem has been addressed in the prior art by adding an alkaline salt to the central plies of the paper to prevent the alum from acting as a mordant. The rosin size can not properly impart water resistant to the paper so the high suction properties of the paper are maintained.xe2x80x94See U.S. Pat. No. 3,300,371 to Hart.
U.S. Pat. No. 2,806,811 to von Hazmburg describes an improved gypsum-core plasterboard, having paper cover sheet containing a strength-imparting resin, such as a melamine-aldehyde condensation product or a urea-aldehyde resin to impart a greater degree of wet strength to the plies. The resins described by von Hazmburg, such as the melamine-aldehyde condensation products or the urea-aldehyde resins, are compatible with the acid paper sizing systems based on rosin and alum that were used in the 1960s, but that those resins do not work for the present neutral paper sizing systems that have a pH between 6 and 8. Moreover, the melamine-aldehyde condensation products are no longer used because the free formaldehyde inherently in the melamine-aldehyde condensation products is ultimately released from the gypsum board panel as a gas. Formaldehyde is a carcinogen and the use of this chemical is restricted in the air-tight houses manufactured today. The prior art does not suggest, however, the addition of cationic polyamide wet strength resins to the backing paper of a gypsum board in order to address the roll-up problem to which the present invention is addressed.
It is, therefore, an object of the present invention to provide a gypsum board which has improved resistance to delamination between the plies of the cover sheets, e.g. improved resistance to xe2x80x9croll upxe2x80x9d.
It is yet another object of the present invention to provide a novel gypsum board in which the strength of the wet bond between the plies of the paper and the bond between the paper cover sheets and the gypsum core is greatly improved.
It is therefore one of the objects, of the present invention to provide a suitable paper cover sheet for gypsum covered wallboards which is of such a nature that it will resist delamination between the plies and resist splitting off the ply directly adjacent the core.
This object is attained by the incorporation with the furnish from which, at least the cover sheets is made of certain strength-imparting resinous additions, which give the cover sheets a greater degree of wet-strength, so that plies within the cover sheets and the plies adjacent the core will resist delamination and splitting, whereby all difficulties from xe2x80x9croll-upxe2x80x9d are minimized or avoided.
The present invention provides a paper covered gypsum board in which the cover sheets have improved resistance to delamination problems such as xe2x80x9cRoll-upxe2x80x9d. A gypsum wall board with improved resistance to xe2x80x9cRoll-upxe2x80x9d is produced by adding a small amount of a cationic wet strength polyamide resin to at least one of the paper cover sheets. In the preferred embodiment, the paper cover sheet includes at least some plies that are made with a neutral size such as alkenyl succinic anhydride (ASA), alkyl ketene dimer (AKD) or neutral rosin and the polyamide resin is added to the paper stock used to produce those plies. Preferably the polyamide resin is added to the paper furnish used to produce all of the plies of the backing paper
Preferably the polyamide resin is a polyamide epichlorohydrin resin. The addition of even small quantities (as little as 0.1% by weight) of a polyamide resin prevents the loss of physical properties (strength, tensile) that lead to roll-up problems and other delamination problems. Conventional wet strength resins, such as melamine formaldehydes and urea formaldehydes, are not suitable when the gypsum board paper is sized with either an alkaline size or a neutral size such as alkenyl succinic anhydride (ASA) or alkyl ketene dimer (AKD).
For purposes of the present invention xe2x80x9cgypsum wall boardxe2x80x9d is defined as a gypsum based core encased in a fibrous envelope including a front cover sheet that covers the front of the board and a back cover sheet that covers the back of the board. The cover sheets are multi-ply paper fiber laminates, usually having 6 or 7 plies, wherein the back cover sheet frequently has a different composition than the front cover sheet. For purposes of the present invention xe2x80x9cRoll-Upxe2x80x9d is defined as a post-manufacturing problem that involves delamination between one or more of the paper plies of the paper cover sheets, and usually the backing paper, but may also be the face paper, especially when the board is handled in high humidity and high temperature environments. For purposes of the present invention, a neutral size is defined as a sizing system that has a pH in the range of about 6.0 to 8.0, as distinguished from conventional rosin/alum sizes that have a pH of about 4.5. In this application, the level of additives used are defined by xe2x80x9cpounds per tonxe2x80x9d which means the number of pounds of the xe2x80x9cactivexe2x80x9d additive per ton of xe2x80x9cdryxe2x80x9d paper solids.